The efficiency of an optical fiber as a signal transmitter depends to a large extent on a precise alignment of its input end with an associated light source to maximize the transfer of luminous energy therebetween. Thus, the maximum-radiance axis of the source--usually a light-emitting diode--must coincide as closely as possible with the axis of the illuminated fiber extremity; when that extremity ends in a spherically convex tip acting as a converging lens, its distance from the source is also critical since its focus should lie at a point of high luminous intensity, generally the center of the source.
If, for example, a light source with near-field emission configuration of 50% at a distance of 23.mu. and 10% at 42.mu. is to be coupled with an optical fiber whose core has a stepped refractive-index profile and a diameter of 80.mu., and if the input end of the fiber has a spherical curvature with a radius of 57.mu. and with a numerical aperture of 0.162, a loss of 3 dB corresponds to a lateral offset of 15.mu. and to a distance variation of 50.mu. from an optimum position.